The lithium-ion battery market has undergone dynamic growth ever since Sony introduced the first commercial cell in 1991. With their energy densities exceeding 130 Wh/kg and cycle lifetimes of more than 1,000 cycles, lithium-ion battery systems have become increasingly popular in applications such as portable computers, camcorders, and cellular phones. J. Power Sources 70 (1998) pp. 48-54. Lithium-ion batteries are the preferred power source for most portable electronic applications because of their higher energy density, longer cycle life, and higher operational voltage as compared with nickel-cadmium and nickel metal hydride batteries. Lithium-ion batteries also provide advantages when compared with lithium batteries in that they generally use a carbon-based anode as opposed to highly reactive metallic lithium. The carbon anode functions via the intercalation of lithium ions between graphene sheets. The cathode materials most commonly used in lithium-ion batteries are lithium cobalt oxide and lithium manganese oxide.
Lithium-ion batteries are produced in spiral wound and prismatic configurations in which a separator is sandwiched between anode and cathode ribbons. The pores of the separator are then filled with an ionically conductive electrolyte formed by dissolving a salt, for example LiPF6, in an organic solvent, for example, 50:50 ethylene carbonate:dimethyl carbonate. The principal functions of the separator are to prevent electronic conduction (i.e., shorts) between the anode and cathode while permitting ionic conduction via the electrolyte. As such, a preferred separator is insoluble in the organic solvent, chemically stable against the electrolyte and electrode active materials, and includes suitably sized pores in its matrix.
The introduction of the rechargeable lithium-ion battery precipitated a need for separators that did more than serve as inert, porous films. Separators were not only required to provide good mechanical and electrical properties, but they also had to incorporate a thermal shutdown mechanism for improved cell safety under conditions of high temperature, overcharge, or physical penetration of the battery can. Specifically, preferred separators have a so-called “fuse effect” in which upon reaching a specified temperature the separator becomes molten and the separator pores collapse to minimize ionic conduction between the electrodes should the battery overheat or short circuit. The “fuse effect” prevents potential ignition of the electrolyte and explosion of the battery.
Most commercially available lithium-ion batteries include microporous polyolefin separators. Such separators are typically made from polyethylene (PE), polypropylene, or some combination thereof because polyolefins provide excellent mechanical properties and chemical stability at a reasonable cost. Because many polypropylene separators have a shutdown temperature that is too high (>160° C.) for use in lithium-ion applications, PE separators are preferred. However, certain polyolefin separators often do not fully shutdown, because of “hole formation” that results from shrinkage and poor mechanical integrity under pressure and high temperature. In contrast, ultrahigh molecular weight PE (UHMWPE)-based separators have good high temperature integrity and sufficient polymer flow to cause pore collapse, thereby preventing current flow between the electrodes and through the separator. Consequently, UHMWPE-based separators are transformed into a nonporous film upon shutdown. UHMWPE-based separators often include linear low density PE (LLDPE), high density PE (HDPE), low density PE (LDPE), or another form of PE to manipulate the shutdown temperature or shutdown rate without compromising mechanical integrity. In the case of a PE microporous separator, the fuse temperature, i.e., the temperature at which the thermal shutdown is effected, is between about 120° C. and about 150° C. Thus separators containing UHMWPE offer desirable safety features for use in lithium-ion batteries.
Significant research has been done to perfect the shutdown mechanism of the battery separator. However, little research has been done on methods to scavenge adventitious moisture or acid from lithium or lithium-ion batteries. The presence of water molecules and certain acids (e.g., hydrofluoric acid (HF)) in lithium or lithium-ion batteries is detrimental to battery performance because the water or acid can react with the dissolved salt in the electrolyte, the anode, the cathode, or the battery can. These side reactions inhibit battery performance.
It is therefore desirable to provide for use in lithium or lithium-ion batteries a microporous polyolefin separator exhibiting excellent mechanical strength and electrical resistance properties, a thermal shutdown mechanism, and water- or acid-scavenging capability.